@article{RothRungEsenetal.2020, author = {Roth, Gian-Luca and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Microchannels inside bulk PMMA generated by femtosecond laser using adaptive beam shaping}, series = {Optics Express}, volume = {28}, journal = {Optics Express}, number = {4}, pages = {5801 -- 5811}, year = {2020}, abstract = {In this contribution, we report on the generation of internal microchannels with basically unlimited channel length inside of PMMA bulk material by femtosecond laser. A precisely controllable and stable circular channel cross section is obtained by using a spatial light modulator to compensate the writing depth depending spherical aberration. Furthermore, the generation of a rotatable elliptical input beam by adaptive optics ensures a fitting of the beam shaping to the writing direction. In this study, we report on both, the effect of the ellipticity of the input beam and the effect of a correction of the spherical aberration on the circularity of the resulting internal microchannels. Moreover, we demonstrate the application of this writing technique by creating microfluidic testing structures inside of a transparent standard polymer.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothEsenHellmann2019, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {A New Approach to Seal Polymer Microfluidic Devices Using Ultrashort Laser Pulses}, series = {Journal of Laser Micro/Nanoengineering}, volume = {14}, journal = {Journal of Laser Micro/Nanoengineering}, number = {1}, pages = {49 -- 53}, year = {2019}, abstract = {We report on the joining of two different transparent thermoplastic polymers using infrared femtosecond laser pulses. Solely being based on nonlinear absorption, the developed micro-welding process for cyclo-olefin copolymers and polycarbonate does not require any intermediate absorbing layers or any surface pre-processing of the welding partners. In addition, it circumvents any heat damages at the outer surfaces of the welding partners. Femtosecond laser welding of cyclo-olefin copolymers is improved by increasing the numerical aperture of the focusing setup resulting in a wider and more homogenous welding cross-section. Welding results are classified by shear strength and fracture modes are analyzed using scanning electron microscopy. Furthermore, we demonstrate, to the best of our knowledge, first results of ultrashort pulse laser welding of transparent polycarbonate, one of the most used transparent polymers. In view of an optimized and stable micro-welding process, we study the influence of laser power and focal position on welding results. The application of ultrashort pulse laser welding for Lab-on-chip fabrication technologies is demonstrated by sealing an ultrashort pulse laser ablated microfluidic device based on a transparent polymer substrate.}, subject = {Femtosekundenlaser}, language = {en} } @inproceedings{RothWolfEsenetal.2019, author = {Roth, Gian-Luca and Wolf, Bastian and Esen, Cemal and Hellmann, Ralf}, title = {Generation of internal 3D microfluidic architectures in polymers by fs laser processing}, series = {Lasers in Manufacturing 2019}, booktitle = {Lasers in Manufacturing 2019}, year = {2019}, abstract = {Microfluidic lab on chip systems require components to transport, mix, separate and analyse small volumes of different fluids. In this study, we report on the laser generation of internal hollow architectures created by focused 514 nm femtosecond laser pulses inside PMMA bulk material. Size and cross-sectional shape of a single internal generated microchannel are determined by the intensity distribution inside the focal voxel and can be controlled either by the numerical aperture of the focusing objective or by laser beam shaping. As both approaches are practically limited with respect to the realizable cross-sectional shapes, we present a process based on an internal hatching to expand the achievable channel cross-sections and thereby enable the possibility to create complex 3D shaped internal structures. This process is applied to create fully internal functional microfluidic elements such as mixers which are part of most polymer lab-on-chip systems.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothEsenHellmann2018, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Laser beam microwelding of transparent plastics}, series = {Joining Plastics}, volume = {12}, journal = {Joining Plastics}, number = {3/4}, pages = {2 -- 8}, year = {2018}, abstract = {The laser beam welding of transparent plastic components is still a challenge according to the state of the art. A new approach for the joining of two transparent polymeric components using ultrashort laser beam pulses is presented in this report. In this respect, the high peak pulse intensity associated with these lasers permits non-linear absorption processes and thus the production of a locally well-defined heat source in the interior even of a transparent material or at the boundary layer between two transparent substrates. Welding results for transparent plastics of the cycloolefin copolymer and polycarbonate types are shown in this study. In addition to the investigation into the influences of fundamental process parameters on the weld geometry, the leak tightness is demonstrated in this study using a microfluidic system.}, subject = {Laserschweißen}, language = {en} } @article{RothEsenHellmann2017, author = {Roth, Gian-Luca and Esen, Cemal and Hellmann, Ralf}, title = {Femtosecond laser direct generation of 3D-microfluidic channels inside bulk PMMA}, series = {Optics Express}, volume = {25}, journal = {Optics Express}, number = {15}, pages = {18442 -- 18450}, year = {2017}, abstract = {We report on laser direct generation of 3D-microchannels for microfluidic applications inside PMMA bulk material by focused femtosecond pulses. Inner lying channels with cross sectional areas from 100 µm2 to 4400 µm2 are directly created in the volume of a PMMA substrate. Using the presented process, the channel length is fundamentally unlimited. Here we demonstrate a channel length of 6 meters inside a substrate with dimensions of 20 × 20 × 1.1 mm. The formation of the micro channels is based on nonlinear absorption around the focal volume that triggers a material modification. The modified volume can be selectively opened to form the channel by a subsequent annealing process. The cross section of the channel is strongly influenced by the energy distribution and illumination around the focal volume determined by the optical setup and process design. The 3D channel layout can easily be realized by moving the specimen using 3D motorized stage, allowing freely chosen complex shaped channel architectures. Within a comprehensive parameter study, varying laser power, number of multi-passes, writing speed and writing depths, we identify an optimized process in terms of attainable channel height, width and aspect ratio, as well as process stability and reproducibility. The proof of concept for an application in three dimensional microfluidic systems is provided by florescence microscopy using a dye rhodamine B solution in isopropanol.}, subject = {Femtosekundenlaser}, language = {en} } @article{RungBokanSchwarzetal.2020, author = {Rung, Stefan and Bokan, Kevin and Schwarz, Simon and Esen, Cemal and Hellmann, Ralf}, title = {Influence of laser wavelength on the modification of friction between 100Cr6 steel and polytetrafluoroethylene by femtosecond laser-induced periodic surface structures}, series = {Journal of Laser Applications}, volume = {32}, journal = {Journal of Laser Applications}, number = {3}, doi = {10.2351/7.0000036}, pages = {032017-1 -- 032017-6}, year = {2020}, subject = {Femtosekundenlaser}, language = {en} } @article{RothHesslerKeferetal.2020, author = {Roth, Gian-Luca and Hessler, Steffen and Kefer, Stefan and Girschikofsky, Maiko and Esen, Cemal and Hellmann, Ralf}, title = {Femtosecond laser inscription of waveguides and Bragg gratings in transparent cyclic olefin copolymers}, series = {Optics Express}, volume = {28}, journal = {Optics Express}, number = {12}, doi = {10.1364/OE.388364}, pages = {18077 -- 18084}, year = {2020}, abstract = {We report on a femtosecond laser based fabrication technique that enables simultaneous single-step generation of optical waveguides and Bragg gratings inside bulk cyclic olefin copolymers. Due to the nonlinear absorption of focused and spatially modulated laser radiation with a wavelength of 514 nm and a pulse duration of 450 fs, a modification concluding a refractive index shift increase inside the substrate can be achieved. A sophisticated characterization of the generated waveguides by means of an elaborate cut-back method reveals a maximum attenuation of 3.2 dB/cm. Additionally, a Mach-Zehnder interferometer is used to examine the waveguide's refractive index profile. The integrated Bragg grating structures exhibit reflectivities up to 95 \% and a spectral full width at half maximum of 288 pm, at a Bragg wavelength of 1582 nm, whereas the grating period can be deliberately chosen by adapting the fabrication parameters. Thus, due to its increased flexibility and the resulting dispensability of cost-intensive phase masks, this method constitutes an especially promising fabrication process for polymer Bragg gratings inside of bulk materials.}, subject = {Femtosekundenlaser}, language = {en} } @article{RothHaubnerKeferetal.2020, author = {Roth, Gian-Luca and Haubner, Julian and Kefer, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Fs-laser based hybrid micromachining for polymer micro-opto electrical systems}, series = {Optics and Lasers in Engineering}, volume = {137}, journal = {Optics and Lasers in Engineering}, number = {10362}, doi = {10.1016/j.optlaseng.2020.106362}, pages = {1 -- 8}, year = {2020}, abstract = {We report on femtosecond laser direct writing of electrically conductive copper structures on transparent cyclic olefin copolymer based planar optical chips. The process is based on a laser-induced reduction of CuO nanoparticles dissolved in a water-soluble resin. Thus generated conductive copper structures are characterized with respect to their chemical composition and electrical resistivity. In addition, the application of ultrashort laser pulses enables a hybrid micromachining approach comprising ablation of polymer substrates and the fabrication of copper patterns in a single laser processing setup. A possible electro-optical application of this approach is demonstrated by employing the copper structures as an electro-thermal microheater in combination with a polymer planar optical Bragg grating sensor. This, in turn, highlights the potential of generating copious micro-opto-electro-mechanical polymer structures for numerous sensing applications ranging from Lab-on-Chip to environmental applications using the presented hybrid micromachining approach.}, subject = {Femtosekundenlaser}, language = {en} } @article{SchwarzRothRungetal.2020, author = {Schwarz, Simon and Roth, Gian-Luca and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Fabrication and evaluation of negative axicons for ultrashort pulsed laser applications}, series = {Optics Express}, volume = {Optics Express}, journal = {Optics Express}, number = {28}, pages = {26207 -- 26217}, year = {2020}, subject = {Ultrakurzzeitlaser}, language = {en} } @inproceedings{SchwarzRungEsenetal.2020, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {Manufacturing of optical elements by non-contact laser processing}, series = {Seventh European Seminar on Precision Optics Manufacturing, Proceedings}, booktitle = {Seventh European Seminar on Precision Optics Manufacturing, Proceedings}, number = {11478}, year = {2020}, subject = {Laserbearbeitung}, language = {en} } @article{ZettlKlarEsenetal.2020, author = {Zettl, Julian and Klar, Maximilian and Esen, Cemal and Hellmann, Ralf}, title = {Generation of Rotationally Symmetric Micro Tools using Ultrashort Laser Pulses}, series = {Journal of Laser Micro/Nanoengeneering}, volume = {15}, journal = {Journal of Laser Micro/Nanoengeneering}, number = {2}, doi = {10.2961/jlmn.2020.02.2007}, pages = {118 -- 122}, year = {2020}, abstract = {We report on the fabrication of rotationally symmetrical geometries with ultrashort laser pulses impinging the constant rotating workpiece tangentially. This particular micro machin-ing of geometry is referred to as laser turning, upon which the laser beam is, in general, orient-ed perpendicularly to the rotational axis and tangentially to the work piece. The target geome-try is realized by moving the constantly rotating specimen according to the specified geometry along the focused laser spot. In contrast to mechanical turning, laser turning induces a mini-mized amount of lateral forces through friction that might deteriorate the geometry. In this study, laser turning is examined for stellite as a function of pulse energy and pulse length in the range between 240 fs and 10 ps. Shorter pulse lengths and higher pulse energies increase the achievable ablation rate, while in the pulse length regime of below 1 ps lower roughness is achieved. With a maximum ablation rate of 1.27 mm³/min and a minimum roughness of 0.17 µm, laser turning combines both, fast processing and high surface quality.}, subject = {Ultrakurzzeitlaser}, language = {en} } @article{SchleierAdelmannEsenetal.2017, author = {Schleier, Max and Adelmann, Benedikt and Esen, Cemal and Hellmann, Ralf}, title = {Cross-correlation-based algorithm for monitoring laser cutting with high-power fiber lasers}, series = {IEEE Sensors Journal}, volume = {18}, journal = {IEEE Sensors Journal}, number = {4}, doi = {10.1109/JSEN.2017.2783761}, pages = {1585 -- 1590}, year = {2017}, subject = {Laserschneiden}, language = {en} } @inproceedings{SchwarzRungEsenetal.2020, author = {Schwarz, Simon and Rung, Stefan and Esen, Cemal and Hellmann, Ralf}, title = {All laser-based fabrication of optical elements}, series = {Proceedings of SPIE 11270, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XX}, booktitle = {Proceedings of SPIE 11270, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XX}, year = {2020}, subject = {Ultrakurzzeitlaser}, language = {de} } @inproceedings{SchwarzEsenHellmann2018, author = {Schwarz, Simon and Esen, Cemal and Hellmann, Ralf}, title = {CO2 laser polishing of femtosecond shaped micro-optical components}, series = {3rd Conference on Laser Polishing LaP 2018}, booktitle = {3rd Conference on Laser Polishing LaP 2018}, year = {2018}, subject = {Kohlendioxidlaser}, language = {en} }